In a photostimulable phosphor imaging system, as described in U.S. Pat. No. RE 31,847 reissued Mar. 12, 1985 to Luckey, a photostimulable phosphor sheet is exposed to an imagewise pattern of high energy, short wavelength radiation, such as x-radiation, to record a latent image pattern in the photostimulable phosphor sheet. The latent image is read out by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photostimulable phosphor destructively releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. The term "destructively" is used herein to denote that the photostimulable phosphor is discharged by the stimulating light, and that only a finite amount of stimulated radiation is emitted by the phosphor, regardless of the quantity of stimulating radiation applied. The term "photostimulable phosphor" as used herein, refers to phosphors that destructively release emitted radiation. To produce a signal useful in electronic image processing, the photostimulable phosphor sheet is scanned in a raster pattern by a beam of light produced, for example, by a laser deflected by an oscillating or rotating scanning mirror. The emitted radiation is sensed by a photodetector such as a photomultiplier tube to produce the electronic signal.
In the conventional photostimulable phosphor imaging system, the photostimulable phosphor sheet is turbid (i.e. scattering) to both stimulating and emitted wavelengths of light. The resolution of such a system is determined to a large extent by the degree to which the laser beam scatters (spreads) within the screen, Hence, in general, a thicker turbid screen will result in a lower system resolution than a corresponding thinner screen. Furthermore, in such a turbid phosphor system, the minimum effective stimulating beam size that can be achieved, and hence the maximum resolution of the system is inversely related to the signal gain. The signal gain cannot be made too small without limiting the signal-to-noise ratio achievable by the system. In the turbid phosphor system, if the stimulating beam power is increased to increase the gain (i.e. the strength of the signal recovered from the photostimulable phosphor), the effective stimulating beam size increases due to scattering of the stimulating beam in the turbid phosphor. FIG. 2 is a plot of the signal gain versus exposure for a photostimulable phosphor imaging panel. The exposure .eta. is directly proportional to the scanning beam power P.sub.o and inversely proportional to the scanning beam velocity v. As shown in FIG. 2, as the exposure increases, the signal increases until the system saturates, i.e. all of the available signal is read out of the photostimulable phosphor. Due to the interrelation between beam power and effective beam size, conventional turbid phosphor imaging systems are sometimes operated in the rising part of the curve to maximize the resolution of the system.
It has been proposed that the resolution of a photostimulable phosphor imaging system may be greatly improved by making the photostimulable phosphor sheet transparent to stimulating radiation, thereby decoupling the effective beam size from the phosphor thickness and hence allowing a maximum resolution that is determined solely by the actual scanning beam size. The term "transparent" as used herein means substantially non-scattering to stimulating radiation. See Canadian Pat. No. 1,175,647 issued Oct. 9, 1984 to DeBoer and Luckey. The object of the present invention is to further improve the resolution of transparent photostimulable phosphor imaging systems of the destructive read-out type.